"Thy words were found, and I did eat them;" (Jeremiah 15:16) to eat is to be sustained, to be nurished by what you're eating. Jeremiah is saying here that in the midst of his persecution, in the midst of his hardship, in the midst of what must have seemed like a spiritual wilderness he found God's Word and was nurished by it.
But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God. (Matthew 4:4)
He continues, "and thy word was unto me the joy and rejoicing of mine heart: for I am called by thy name, O LORD God of hosts." Here we see Jeremiah rejoicing because he knows the Word. It is in the Word of God that we find God's love, patience, righteousness, and glory displayed (at least until the return of Christ!) most clearly. He knows the Word and because of that knowledge he rejoices. He's being persecuted, people hate him because he is a reproach to their sinful lives and still he finds joy! It is in knowing and embracing the Word of the Lord that we can rejoice when those around us who do not know the Lord would be weeping and crying out about their misfortune.
We rejoice in knowing that we are "called by thy name, O LORD God of hosts." Ignorance of God's Word often leads to doubt, sadness, and hopelessness. But as we learn and grow in the Lord, when we come to understand this God who knew us before we were born, this God who guides and protects us, this same God who created the heavens is calling us to Him we can rejoice and experience a deep setting joy and peace that is not dependant upon our current situations. It is in knowing God as revealed through His Word that we find this joy. And I rejoice in the fact that He cares so much for us that He has chosen to give us His Word as a constant reminder of His ever loving presence in the lives of those who serve Him.